1. Field of Industrial Application
The present invention relates to a planar type light valve drive substrate device to be used in a direct viewing type display system or a projection type display system.
More specifically, the present invention relates to a semiconductor integrated circuit substrate device which has a semiconductor thin film coating a substrate surface and formed with a pixel electrode group, a switch element group and a drive circuit element group. This substrate device is packaged integrally with a liquid crystal panel, for example, to constitute the so-called "active matrix device".
2. Prior Art
The active matrix device is based upon a simple principle, in which pixels are individually equipped with switch elements. If specific pixels are selected, the corresponding switch elements are turned on. If no pixel is selected, the switch elements are left nonconductive. In addition, the individual switch elements are driven by peripheral circuit elements constituting the driver circuit. These switch elements and peripheral circuit elements are formed over a glass substrate which forms part of the liquid crystal panel.
Therefore, the technology for making the switch elements and the peripheral circuit elements thin is important. Thin film transistors are usually formed as those elements.
In the active matrix device of the prior art, the thin film transistors are are formed in the surface of an amorphous silicon thin film or a polycrystal silicon thin film, which is deposited on the glass substrate. These amorphous silicon thin film and polycrystal silicon thin film are suited for fabricating an active matrix device of relatively large frame, because they can be easily deposited over the glass substrate by using the vacuum evaporation process or the chemical vapor deposition process. Thus, the active matrix device is suited for the direct viewing type display device.
In recent years, however, there arises a higher demand not for the direct viewing type display device but for a high-speed miniature display device having miniature pixels of high density. That miniature light valve device is used as a plane for forming a primary image of a projection type image device, for example, so that it can be applied as a projection type high-vision TV. For this application, it is desirable to provide a semiconductor integrated substrate device for a high-speed miniature light valve, which has a pixel size of the order of 1 .mu.m and an overall size of several cm by using the miniature semiconductor fabricating technology.
If, however, the existing amorphous thin film is used, the ON current density is too low to operate at a high speed because the material is not made of a single crystal. Moreover, the transistor elements of sub-micron order cannot be formed by applying the miniature semiconductor technology. In the case of the amorphous silicon thin film, for example, its film forming temperature is about 300.degree. C. so that a high-temperature treatment necessary for the miniaturization cannot be executed. In the case of a polycrystal silicon thin film, on the other hand, the crystal particles have a size of several .mu.m which raises a problem in that the miniaturization of the transistor elements is restricted.
In the semiconductor integrated circuit substrate device for the existing active matrix display device using an amorphous material, as has been described hereinbefore, there arises a problem in that it is seriously difficult to realize an integration density, a high-speed operation and a chip size similar to those of the ordinary semiconductor integrated circuit elements.
In order to reduce the size of the semiconductor integrated circuit substrate device, it is especially necessary to integrate a peripheral circuit element group in a remarkably high density in addition to the switch element group. It is, however, difficult to form the peripheral circuit element group requiring a more miniature like technology in a super-high density in the polycrystal silicon thin film or the amorphous silicon thin film. As a result, it has been impossible to realize a semiconductor integrated circuit substrate device for the active matrix device, which has a size substantially equal to that of the ordinary LSI chip.
In view of the aforementioned problems of the prior art, a first object of the present invention is to provide a structure of and a process for fabricating a semiconductor substrate device, which can form a peripheral circuit element group requiring a higher speed and a higher-density integration over a substrate shared with a switch element group for supplying electric power selectively to pixels.
Since, moreover, the silicon single crystal thin film used as the semiconductor single crystal and the transparent electrically insulating substrate have different coefficients of thermal expansion, troubles such as separation or cracking occur in the course of the LSI fabrication process requiring a temperature as high as 800.degree. C. Incidentally, the silicon single crystal film has a coefficient of thermal expansion of 3.6.times.10.sup.-6 /.degree.C., and the transparent insulating substrate has, if made of quartz, a coefficient of thermal expansion of 0.4.times.10.sup.-6 /.degree.C.
In order to eliminate this difficultly, a second object of the present invention is to provide a semiconductor single crystal composite substrate which has a high quality and a high production efficiency by interposing an intermediate layer between the silicon single crystal thin film and the transparent substrate to minimize the separation and cracking of the two due to thermal shock.
If the switch elements of the pixel array unit are miniaturized, their breakdown voltage raises a problem. Specifically, the light valve device or the active matrix device has its individual pixels fed with drive signals at a relatively high voltage. Therefore, the switch elements for supplying the electric power to the individual pixels selectively also have to withstand such high-voltage drive signals. Therefore, a third object of the present invention is to provide a light valve substrate semiconductor device, in which switch element MOSFETs having a special high breakdown voltage are integrated in a high density structure.
On the other hand, the pixel electrode material used in the prior art is exemplified by a transparent conductive thin film such as an ITO film or a NESA film. This transparent conductive thin film can be deposited relatively easily by the vacuum vapor deposition process or the sputtering process but is defective in low heat resistance and has poor patterning precision due to an etching process.
Thus, this material has a poor coordination with the LSI fabrication technology requiring the high-temperature treatment and is accompanied by a problem that the semiconductor process cannot be consistently used. Another problem is that the material is not suited for making the pixels miniature and highly dense because of the poor patterning precision. If, moreover, the roughness of the substrate surface grows relatively serious as the switch element groups are integrated in a high density, the transparent conductive thin film to be formed thereover is stepwise cut to raise a problem that the defect percentage of the pixels grows high. Therefore, a fourth embodiment of the present invention is to provide a light valve substrate semiconductor device having a miniature pixel electrode structure, to which the LSI fabrication technology or the semiconductor process can be consistently applied up to the final step.
In the active matrix device, moreover, a predetermined charge is supplied during a selection period to the pixel electrodes through the switch elements, and the charge supplied is retained during the unselected period in the pixel electrodes to execute the light valve functions for each pixel. If, at this time, the switch elements are formed in the semiconductor single crystal thin film in accordance with the general object of the present invention, its optical dark current is larger than that of the thin film transistors which are formed in the amorphous silicon thin film or the polycrystal silicon thin film. If no counter-measure is taken, the stored charge is allowed to leak due to the high optical dark current during the unselected period occupying a major part of one frame, thus raising a problem that the voltage to be applied to the pixels will drop. Therefore, a fifth object of the present invention is to provide a light valve substrate single crystal thin film semiconductor device which is enabled to effectively prevent the drop of a voltage to be applied to the pixels even if silicon single crystal thin film transistor elements having a relatively high optical dark current are used.
Generally speaking, the light valve device semiconductor substrate has an optically transparent region, which is formed with a switch element group for energizing a pixel electrode group and each pixel electrode selectively, and an optical opaque region which is formed with a peripheral circuit containing a circuit element group for driving the switch element group. In each region, the switch element group and the circuit element group are individually separated electrically by the element separation region. Incidentally, the optically opaque region is required to have a sufficient optical transparency even in the element separate region so as to enhance the optical transparency but is allowed to have a margin for its sizing precision. In the opaque region, on the contrary, the grouped circuit elements such as the transistors constituting the peripheral circuit are integrated in a high density so that the element separate region is required to have fine and highly precise sizing and shaping controls but need not pass any incident light. Thus, the opaque region is preferred to be optically opaque. In view of this point, a sixth object of the present invention is to improve the performance of the whole light valve semiconductor integrated circuit device by forming an element separate region which has different sizing and shaping precisions and different optical characteristics for the optically transparent region and the optically opaque region.
In the existing semiconductor device using the SOI (i.e., Silicon Oxide Insulator) substrate, the diodes used in a protection circuit for the input/output terminals are PN junction diodes. In this case, the diodes have a small junction area because they are formed in a thin film. This is because the impurity region has a thickness as deep as the thin film. This makes it difficult to raise the dielectric withstand voltage.
In view of the aforementioned problems of the prior art, a sixth object of the present invention is to provide a semiconductor device using the SOI substrate with a protection circuit having a high dielectric breakdown voltage.
In the prior art, on the other hand, there are known various types of semiconductor laminated substrates, in which a semiconductor layer is formed over such carrier layer, as is so-called the "SOI substrate". This SOI substrate is prepared by depositing a polycrystal silicon thin film on a carrier surface made of an insulating material, for example, by using the chemical vapor deposition process and then by re-crystallizing the polycrystal film into a re-crystallized structure by irradiating a laser beam. Generally speaking, however, the single crystal prepared by re-crystallizing a polycrystal does not always have a uniform azimuth but has a high lattice defect density. For these reasons, it is difficult to apply the miniaturization technology such as used with the silicon single crystal wafer to the SOI substrate fabricated by the process of the prior art or to package photo voltaic energy elements of high performance. In view of this point, a seventh object of the present invention is to provide a light valve device which packages photo voltaic energy elements of fine and high resolution by using a semiconductor thin film having a crystal azimuth as uniform as that of the silicon single crystal widely used in the semiconductor process and a lattice defect of as low density as the same.
On the other hand, the electrooptical modulation substance to be generally used in the light valve device of the prior art is a liquid crystal. This liquid crystal is confined between a pair of substrates opposed at a predetermined gap to each other and has its molecules arrayed in a predetermined direction. In order to realize this arrayed state of the liquid crystal molecules, the inner surfaces of the substrates are subjected to the so-called "alignment". This alignment is generally accomplished by rubbing the substrate surfaces with cotton cloth.
However, the foregoing general objects of the present invention are troubled by the following problem. In case of element integrations of high density, the roughness of the surface of the semiconductor thin film substrate is serious with respect to the pixel size so that the alignment cannot be executed by the rubbing treatment of the prior art. In other words, the roughness of the substrate surface makes the uniform rubbing treatment difficult which degrades the quality of the displayed image. Another problem is that the switch elements miniaturized to a sub-micron order may be broken by the rubbing treatment. Moreover, the blots or dust generated by the rubbing treatment may be larger than the pixel size causing a problem in that the optical transparency of the pixels is reduced.
In view of the aforementioned problems caused by the rubbing treatment of the prior art, an eighth object of the present invention is to provide an active matrix type liquid crystal light valve device which has such a liquid crystal alignment structure that does deteriorate the miniaturized switch elements or the pixel electrodes.